Rod Failure - Alberta Oil Tool

A Special Report from Alberta Oil Tool

Failure Mechanisms ……………………………….. 1
Design and Operation Failures ………………….. 3
Mechanical Failures ……………………………….. 4
Bent Rod Failures ………………………………….. 5
Surface Damage Failures ………………………… 5
Connection Failures ……………………………….. 6
Corrosion Failures …………………………………. 9
Acid Corrosion ……………………………………... 10
Chloride Corrosion ………………………………… 10
CO 2 Corrosion ………………………………….…… 10
H 2 S Corrosion ………………………………………. 11
Microbiologically Influenced Corrosion (MIC) ... 11
Oxygen Enhanced Corrosion ……………………. 12
Scale Corrosion ……………………………………. 12
Stray Current Corrosion ………………………….. 13
Manufacturing Defects ……………………………. 13

Root Cause Failure Analysis is Essential for Failure
Frequency Reduction in Wells With Artificial Lift.
Excerpted from Sucker Rod Failure Analysis by: Clayton T. Hendricks and Russell D. Stevens
Most failures associated with artificial lift systems can be attributed to one of three downhole
components - pump, sucker rod, or tubing. A pump, sucker rod, or tubing failure is defined as any catastrophic
event requiring servicing personnel to pull or change-out one or more of these components. By this definition,
the failure frequency rate is the total number of component failures occurring per well, per year. Marginally
producing wells with high failure frequency rates are often classified as “problem” wells and effective failure
management practices can mean the difference between operating and plugging these wells. Failure
management includes preventing, identifying, implementing and recording the “real” root cause of each failure
and is central to overall cost-effective asset management. For the purpose of this photo essay, we will deal only
with sucker rod failures.
Cost-effective failure management begins with prevention, and the time to stop the next failure is nowprior
to an incident! Simply fishing and hanging the well on after a sucker rod failure will not prevent failure
recurrence. In fact, most failures continue with increasing frequency until the entire rod string must be pulled
and replaced. Achievable failure frequency reductions require accurate failure root cause analysis and the
implementation of corrective action measures to prevent failure recurrence. A database capable of querying the
well “servicing” history is needed to tack and identify failure trends. Once a failure trend is identified, remedial
measures should be implemented during well servicing operations to prevent premature rod string failures. The
database failure history should include information on the failure type, location, depth, root cause, and the
corrective action measures implemented.
Sucker rods can be caused to fail prematurely. Understanding the effects of seemingly minor damage
to rod strings, and knowing how that damage can produce catastrophic failures, is very important for production
personnel. Sucker rod failure analysis is challenging and you need to be able to look past the obvious and seek
clues from the not so obvious. All production personnel should have adequate knowledge and training in failure
root cause analysis. Understanding how to identify failures and their contributing factors allows us an
understanding of what is required to correct the root cause of the failure. Every step that can be taken to
eliminate premature sucker rod failures must be taken. On-going training programs concerning sucker rods
should include formal and informal forums that advocate following the recommendations of manufacturers for
artificial lift design, care & handling, storage & transportation, running & rerunning, and makeup & breakout
procedures. A variety of training schools are currently available and, with advanced notice, most can be tailored
to meet the specific needs of production personnel.
Failure Mechanisms
All sucker rod, pony rod, and coupling fractures are either tensile or fatigue failures. Tensile failures
occur when the applied load exceeds the tensile strength of the rod. The load will concentrate at some point in
the rod string, create a necked-down appearance around the circumference of the rod, and a fracture occurs
where the cross-section is reduced. This rare failure mechanism only occurs when pulling too much load on the
rod string- such as attempting to unseat a stuck pump. To avoid tensile failures, the maximum weight indicator
pull for a rod string in “like new” condition should never exceed 90% of the yield strength for the known size and
grade of the smallest diameter sucker rod. For unknown sucker rod conditions, sizes, or grades a sufficient derating
factor should be applied to the maximum weight pulled. All other sucker rod, pony rod and coupling
failures are fatigue failures.
Fatigue failures are progressive and begin as small stress cracks that grow under the action of cyclic
stresses. The stresses associated with this failure have a maximum value that is less than the tensile strength
of the sucker rod steel. Since the applied load is distributed nearly equally over the full cross-sectional area of
the rod string, any damage that reduces the cross-sectional area will increase the load or stress at that point
and is a stress raiser. A small stress fatigue crack forms at the base of the stress raiser and propagates
perpendicular to the line of stress, or axis of the rod body. As the stress fatigue crack gradually advances, the
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mating fracture surfaces opposite the advancing crack front try to separate under load and these surfaces
become smooth and polished from chafing. As the fatigue crack progresses, it reduces the effective crosssectional
area of the sucker rod until not enough metal remains to support the load, and the sucker rod simply
fractures in two. The fracture surfaces of a typical fatigue failure have a fatigue portion, tensile portion, and final
shear tear.
Fatigue failures are initiated by a multitude of stress raisers. Stress raisers are visible or microscopic
discontinuities that cause an increase in local stress on the rod string during load. Typical visible stress raisers
on sucker rods, pony rods and couplings are bends, corrosion, cracks, mechanical damage, threads, and wear
or any combination of the preceding. This increased stress effect is the most critical when the discontinuity on
the rod string is transverse (normal) to the principle tensile stress. In determining the stress raiser of a fatigue
failure, the fatigue portion opposite the final shear tear (extrusion/protrusion) must be carefully cleaned and
thoroughly examined. Fatigue failures have visible or macroscopic identifying characteristics on the fracture
surface, which help to identify the location of the stress raiser. Ratchet marks and beach marks are arguably
two of the most important features in fatigue failure identification. Ratchet marks are lines that result from the
intersection and connection of multiple stress fatigue cracks while beach marks indicate a change in the rate of
crack growth. Ratchet marks are parallel to the overall direction of crack growth and lead to the initiation point
of the failure. Beach marks are elliptical or semi-elliptical rings radiating outward from the fracture origin and
indicate successive positions of the advancing stress fatigue crack growth.
Figure 1
Figure 1 is an example of tensile and fatigue
failures. The two examples on the right are tensile
failures. A tensile failure is characterized by a reduction
in the diameter of the cross-sectional area at the point of
fracture. Typical tensile failures have cup-cone fracture
halves. The second example from the right is typical in
appearance for tensile failures. During the final stages of
an overload, fractures from tensile failures rupture, or
shear, on 45ϒ angles to the stresses applied. A good
example of the shear is the characteristic cup-cone
fracture surfaces of a typical tensile failure. The rod body
on the right is an excellent example of needing to look
past the obvious for the not so obvious. A fatigue failure
is primarily responsible for the failure even though fracture occurred while trying to unseat a stuck pump. Visual
examination of the fracture surface reveals a small semi-elliptical, stress fatigue crack. This sucker rod had preexisting,
transverse fatigue cracks, from in-service stresses. One of the stress fatigue cracks opened during the
straight, steady load applied in attempting to unseat the pump, and fracture occurred. The tensile failure is
secondary and results in the unusual appearance of the fracture surface – with the small fatigue portion, large
tensile portion and unusually large 45ϒ double shear tears.
The remaining examples are fatigue failures on: casehardened sucker rods; normalized and tempered
sucker rods; and quenched and tempered sucker rods. The example on the far left is a torsional fatigue failure
from a progressing cavity pump. Ratchet marks found in the large fatigue portion, and originating from the
surface of the rod body, completely encircle the fracture surface with the small tensile tear portion shown slightly
off middle-center. The second rod body on the left is a casehardened fatigue failure. The case encircling the
rod body diameter carries the load for this high tensile strength sucker rod and if you penetrate the case, you
effectively destroy the load-carrying capability of this type of manufactured sucker rod. The fatigue crack
advances around the case and progresses across the rod body. A fatigue failure on a casehardened sucker rod
generally exhibits a small fatigue portion and a large tensile tear. The third rod body from the left is typical in
appearance for most fatigue failures. Typical fatigue failures have a fatigue portion, tensile portion with a final
shear tear. The width of the fatigue portion is an indication of the loading involved with the fracture. Mechanical
damage can prevent or hinder failure analysis by destroying the visual clues and identifying characteristics
normally found on a fatigue fracture surface. Care must be exercised when handling the fracture halves. It is
very important to resist the temptation to fit the mating fracture surfaces together since this almost always
destroys (smears) microscopic features. To avoid mechanical damage, fracture surfaces should never actually
touch during fracture-surface matching.
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Design and Operation Failures
Sucker rod failure prevention begins with design. It is possible for poorly designed rod strings to
contribute to other component failures in the artificial lift system, such as rod cut tubing resulting from
compressive rod loads. Designing the artificial lift system is a compromise between the amount of work to be
done and the expense of doing this work over a cost-effective period of time. Numerous combinations of
depths, tubing sizes, fluid volumes, pump sizes and configurations, unit sizes and geometries, stoke lengths,
pumping speeds and rod tapers are available to the system designer. Sucker rod size and grade selection is
dependent upon many factors including predicted maximum stresses, stress ranges, and operating
environments.
Figure 2 Commercially available computer design
programs allow the system designer to optimize
production equipment at the least expense for the well
conditions existing at the time of the design. After the
initial design and installation of the rod string, periodic
dynamometer surveys should be utilized to confirm that
equipment load parameters are within those considered
acceptable. A good initial design may become a poor
design if well conditions change. Changes in the fluid
volume, fluid level, stroke length, stokes per minute or
pump size severely impact the total artificial lift system.
Changes in fluid corrosiveness can affect the fatigue
endurance lift of sucker rods and may lead to premature
failures. When one of the preceding conditions change, the design of the artificial lift system must be reevaluated.
Figures 2 and 3 are examples of design and
operationally induced mechanical failures. Wear, flexing
fatigue, unidirectional bending fatigue, and stress-fatigue
failures indicate compressive rod loads, deviated wells,
fluid pound, gas interference, highly stressed sucker rods,
improperly anchored tubing, pumps tagging bottom,
sticking pump plungers, unanchored tubing, or some
combination of the preceding.
Wear causes rod failures by reducing the crosssection
of the metal, exposing new surface metal to
Figure 3 corrosion, and causes joint failures from impact and
shoulder damage. The Class T coupling on the left, the
Class SM coupling second from left, and the rod body on the left are all examples of wear. Wear on the sucker
rod string is defined as the progressive removal of surface metal by contact with the tubing. Wear that is equal
in length, width, and depth usually suggests a deviated or crooked well bore. Angled wear patterns indicate rod
strings that are aggressively contacting the tubing at an angle, usually as a result of fluid pound or unanchored
(improperly anchored) tubing. The middle rod body represents corrosion-abrasion wear. Wear also removes
corrosion inhibiting films and exposes new surface metals to corrosive well fluids-which accelerate the rate of
corrosion. The Class T coupling on the far right has a work-hardened ridge from tubing-slap wear. Tubing-slap
wear is the result of the rod string “stacking out” – probably as a result of fluid pound, gas interference or pump
tagging. The work-hardened material doesn’t wear as fast as the softer material on either side of the workhardened
area, and it leaves a ridge of material as the rest of the coupling wears.
The second rod body from the left is a flexing fatigue failure. Flexing fatigue failures occur from the
motion of the rod string having a constant lateral or side movement during the pumping cycle. Stress fatigue
cracks due to flexing will concentrate along the area of the rod where the greatest bending stresses occurred.
The transverse, stress fatigue cracks will be on one half of the circumference of the rod body, closely spaced
near the rod upsets and gradually spreading apart moving toward the middle of the rod body. Most flexing
fatigue failures occur above the connection in the transition zone of the rod body-between the rigid coupling and
upset area and the more flexible rod body. Flexing fatigue failures will not show permanent bends since this
problem occurs while the rod string is in motion. The example on the far right is a unidirectional bending fatigue
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failure. This type of failure generally has two tips protruding above the fracture surface. These distinct failure
characteristics indicate a double shear-lip tear. Double shear-lip tears are the direct result of unidirectional
bending stresses, with fractures and the fatigue damage occurring under compressive rod loads. Compressive
rod loads may be the result of large bore pumps with small diameter sucker rods or multiple tapers in shallow
wells.
The second rod body sample on the right (Figure 3) is a stress fatigue failure. Stress fatigue failures
occur on highly stressed sucker rods as a result or worn out sucker rods, overloads, or extremely high rod loads
for short periods of time. Stress fatigue failures may have closely spaces, fine, transverse secondary fatigue
cracks that completely encircle the circumference of the rod body. The stress fatigue cracks may be on the
wrench square and over the entire length of the rod body. With very old sucker rods, stress fatigue cracks and
failure may occur within normal everyday operating loads.
Figure 4 is an example of coupling-to-tubing slap.
Coupling-to-tubing slap is the result of extremely
aggressive angle contact to the tubing by the rod string.
This aggressive contact is the direct result of severe fluid
pound, unanchored (or improperly anchored) tubing,
sticking (or stuck) pump plungers, or any combination of
the preceding.
Figure 5
Figure 4
Figure 5 is an example of rod guide related
damage. The example on the left is a reconditioned, high
tensile strength sucker rod. Turbulent fluid flow,
associated with short, blunt-end injection molded rod
guides, allowed crevice corrosion in the critical wash area
around the end of the guide. Prior to inspecting the moldon
rod guides were removed from the rod body for
reconditioning. Class 1 reconditioned sucker rods cannot have discontinuities greater than 20 mils (0.020”) per
API Specification 11BR. The crevice corrosion was under the 20 mils allowed for a Class 1 reconditioned
sucker rod. However, the notch sensitivity (discontinuity intolerance) of a high tensile strength sucker rod is
high. In other words, small pits can be detrimental to the high tensile stresses associated with the high strength
sucker rod and reconditioned high strength sucker rods should be de-rated for load. The example in the middle
is an erosion/corrosion failure resulting from short, blunt-end; field applied rod guides in small tubing with high
fluid velocities. Erosion/corrosion pits will be “fluid cut” with very smooth bottoms. Pit shape characteristics may
include sharp edges and steep sides if accompanied by CO 2 or broad smooth pits with beveled edges if
accompanied by H 2 S. The example on the right is abrasion wear from a field-applied guide moving up and
down on the rod body during the pumping cycle. Generally speaking, mold-on rod guides provide better laminar
flow, a minimum of three to four times more bonding and holding power and are more cost-effective than are
field applied rod guides.
Mechanical Failures
Mechanical failures account for a large percentage of the total number of all rod string failures.
Mechanical failures include every type except manufacturing defects and stress/corrosion fatigue. Mechanical
damage to the rod string contributes to a stress raiser which will cause sucker rod failures. The time to failure
will be influenced by many variables, of which maximum stress, operating environment, orientation of the
damage, sucker rod chemistry, sucker rod heat treatment type, stress range and type of damage will be of the
most important. Mechanical damage can be caused by inept artificial lift design, improper care and handling
procedures, careless makeup and breakout procedures, out-of-date operating practices, or any combination of
these elements.
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Bent Rod Failures
Bending fatigue failures account for a significant number of all mechanical failures. It is a fact that all
bent sucker rods eventually fail. New sucker rods are manufactured to a body straightness of no less than 1/16
inch in any twelve inches of rod body length. Sucker rods within this tolerance of straightness will roll easily on
a level rack with five supports. Any degree of bend greater than this will cause an increase in local stress at the
point of the bend during applied load. When the bent rod body is pulled straight during load, the fatigue strength
of the material is quickly reached. The cycle of continually exceeding the ultimate material strength is repeated
during the pumping cycle and causes stress fatigue cracks on the concave side of the bend. These stress
fatigue cracks progress across the bar, during load, until not enough metal remains in the bar to support the
load, and fracture occurs.
Straightening the raw bar stock is the first step in the process of manufacturing sucker rods. Cold
straightening the bar deforms the grain structure below its recrystallization temperature, putting a strain in the
bar that is accompanied by a work hardening effect. During the manufacturing process, the function of heat
treatment is to stress-relieve the residual and induced stresses caused by bar rolling, bar straightening
processes and from forging the rod upsets. Heat treatment changes the metallurgical structure of the forged
ends to match that of the rod body and also controls the mechanical properties of the sucker rod. Any rod body
bend created after hear treatment causes work hardening, which creates an area of hardness different than the
surrounding surfaces. This condition is referred to as a “hard spot” and is a stress raiser to load. Mechanical
processing, such as passing the finished sucker rod through a system of rollers, will attempt to remove the bend
so it appears to be straight. However, reconditioning processes are not capable of stress relieving bent sucker
rods. A bent sucker rod is permanently damaged and should not be used because all bent sucker rods will
eventually fail.
Figure 6 (with inset of Figure 7) is an example of
binding fatigue failures. Bending fatigue failures can be
identified by the angled fracture surface, which will be at
some angle other than 90ϒ to the axis of the rod body.
The example on the left illustrates a fracture caused by a
long radius bend, or gradual bow in the rod body (left
example in Figure 7). The fracture surface shows fatigue
damage, and has a slight angle when compared to the
axis of the rod body. The middle example is a short
radius bend (right example in Figure 7). The fracture
surface is at a greater angle to the axis of the rod body
with a small fatigue portion and a large tensile tear
portion. The example on the right is the result of a
Figure 6 Figure 7
corkscrewed sucker rod. Notice how convoluted the fracture surface is in
appearance. As a general rule, the greater the bend in the rod body, the more
convoluted the fracture surfaces appear. In operation, the time for the rod to fracture
is greatly shortened. Poor care and handling procedures usually cause bent rods.
Surface Damage Failures
Everything possible should be done to prevent mechanical surface damage
to sucker rods, pony rods and couplings. Surface damage increases stress during
applied loads, potentially causing rod string failures. The type of damage, and its orientation, contributes to this
increased stress effect. The orientation of the damage contributes to higher stresses with transverse damage
having increased stresses over those associated with longitudinal damage. A sharp nick will create a higher
stress concentration and would be more detrimental to load than a shallow, broad-based depression. Sucker
rods with indications of surface damage must not be used and must be replaced. Care should be used to avoid
all metal-to-metal contact that might result in dents, nicks, or scratches. To prevent potential sucker rod
damage, place strips of wood between metal storage racks and between each layer of sucker rods so metal-tometal
contact can be avoided. Use sucker rods for what they were designed for- to lift a load. Never use sucker
rods as a walkway or workbench. Keep metal tools not intended for use on sucker rods and all other metal
objects away from the rods. Make sure the tool you use is intended for the purpose and ensure that it is in
proper working order.
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Figure 8 is an example of various surface
damage failures. The example on the left shows a slight
depression from a wrench, tool, or other metal object.
The second example from the left is damage from a pipe
wrench used in applying field-installed rod guides. The
second example from the right has a small longitudinal
scratch, through metal-to-metal contact, by allowing
sucker rods to run down other rods in a rod bundle during
installation. The example on the right exhibits transverse
surface damage.
Figure 8
Figure 9 is an example of surface damage
caused by sucker rod elevators. The bottom example is
damage from worn or misaligned elevator seats. After an extended period of service, the elevator seats
become so worn that they develop an oval shape rather than a round shape. As the oval shape grows, the
tangency ring of the rod upset to the elevator seat face is
lowered in the front half of the seat. As the seat continues
to wear the seating position of the rod upset is moved
forward of the elevator trunnion centerline. This causes
an offset in the hook load and tilts the elevator body
forward. When the elevator lifts the rod string load, the
hook load will bend the sucker rod centerline to coincide
with the elevator trunnion centerline. As the rod string
weight increases, the hook load will bend every sucker
rod engaged by this elevator. Bent sucker rod failures
that occur below the surface upset bead may be from bad
elevator seats. The top example is damage caused by
the elevator latches. This type of damage normally
occurs as a result of picking up or laying down in doubles.
Never pick up or lay down anything more than one single sucker rod. Anything else causes the elevator latches
to act as a fulcrum and allow bending stresses to concentrate in the transition zone of the rod body and the
forged upset.
Connection Failures
Figure 9
The API sucker rod connection is designed as a shouldered, friction loaded connection. Since the
fatigue endurance of the sucker rod connection is low when subjected to cyclic loads it is necessary to limit the
cyclic loads with pin preload. If the pin preload is greater than the applied load the load in the connection
remains constant and no fatigue occur from cyclic loads. The friction load that develops between the pin
shoulder face and the coupling shoulder face, helps lock the connection together to prevent it from coming
unscrewed downhole. However, if the preload is less than the applied load, the pin shoulder face and the
coupling shoulder face will separate during the cyclic motion of the pumping unit. Once these faces separate
the connection is cyclically loaded and will result in a loss of displacement, or loss of tightness, failure. Loss of
displacement failures may result from improper lubrication, inadequate makeup, over-torque, tubing-slap wear,
or any combination of these elements.
Figure 10 is an example of pin failures due to a loss
of displacement. The sample on the right is typical in
appearance for a loss of displacement pin failure.
Insufficient makeup, or the loss of tightness, caused the pin
shoulder face and the coupling shoulder face to separate.
When these faces separate, a bending moment is added to
the tensile load in the pin. The threaded section of the pin
is held rigid while the rest of the pin flexes. The motion of
the rod string causes stress fatigue cracking to start in the
first fully formed thread root above the undercut. Small
fatigue cracks begin along the thread root and consolidate
into a major stress fatigue crack. The fracture surface of a
typical loss of displacement pin failure has a small fatigue
Figure 10
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portion covering approximately one-third of the fracture surface with the tensile tear portion and final shear tear
covering the remaining fracture surface. The examples on the left and in the middle will occur as a result of
stress loading when stress-raising factors such as corrosion or mechanical damage is present on the surface of
the pin undercut.
Figure 11 is another example of two types of pin
failures. The sample on the left is typical in appearance
for a loss of displacement pin failure. However, this pin
fracture was caused by the hydraulic rod tongs during
makeup as is evidenced by the stair-stepped tensile tear.
It is not uncommon for pin fractures to occur at makeup, if
the pin has a pre-existing stress fatigue crack due to the
high torque required during joint makeup, with large
diameter Class D and all sizes of high tensile strength
sucker rods. The sample on the right is an example of
excessive torque on a soft pin. The fracture surface has a
large fatigue portion, with multiple ratchet marks in the
pin-thread root, and a small tensile portion.
Figure 12 is an example of a loss of displacement
coupling failure. The fracture initiated in the coupling
thread-root opposite the first fully formed pin starting
thread. One-third/two-third fracture halves, in length, with
ratchet marks originating in the thread root indicate a loss
of displacement coupling failure. The fracture surface of a
typical loss of displacement coupling failure has a small
fatigue portion and a large tensile tear portion. Loss of
displacement coupling failures are primarily associated
with Class D sucker rods and high tensile strength sucker
rods.
Figure 11
Figure 12
Mid-length coupling fractures, with ratchet marks
leading from the outside, indicate another type of failure. The fatigue crack starts from the outside coupling
surface, progressing inward toward the threads, then around the coupling wall to a tensile fracture. Mid-length
fractures indicate coupling failures from mechanical damage to the coupling surface, exceeding the stress
fatigue endurance limit of the material, or a manufacturing defect. Most mid-length coupling fractures are the
result of mechanical damage or over load. Mid-length coupling fractures due to overload have a small fatigue
portion and large tensile tear portion. This failure is common with high strength sucker rods and Class SM
couplings. Use Class T couplings to avoid mid-length coupling failures with high tensile strength sucker rods.
Figure 13 is an example of thread galling in the
sucker rod connection. Thread galling is mechanical
damage to the sucker rod and/or coupling threads.
Thread galling is the result of damaged or contaminated
threads causing the interference between the threads to
be great enough to rip and tear the thread surfaces. The
threads weld together during makeup and strip apart at
breakout and the connection is damaged and destroyed
beyond use. Hard stabbing damage to the leading
thread, contaminated threads and cross threading on
larger diameter rods are the primary causes of thread
galling. Cleaning the threads prior to makeup, properly
lubricating the threads and following careful makeup
procedures will prevent thread galling.
Figure 13
Figure 14 is an example of wrench square failures. Wrench square failures are extremely rare and
seldom occur unless from mechanical damage, corrosion, manufacturing defects or torsional stress causing
fatigue damage. The example on the left is a wrench square failure from severe mechanical damage. A loose
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or sloppy backup on the hydraulic rod tongs has rounded
the wrench square corner. The stress fatigue crack
began in the corner of the wrench square and progressed
to final rupture or fracture.
Figure 14
The example on the right is a wrench square
failure from a manufacturing defect. The failure initiated
in the die stamp mark and is an example of an excessive
die stamp depth failure. Die stamp markings can become
notches that serve as stress raisers if the depth of the die
stamping, during the forging process, is not controlled and
kept within API Specification 11B, allowable tolerances.
Figure 15 is an example of the damage that
occurs as a result of severely over-tightening the sucker
rod connection. The example shown is an over-tightened
coupling that has flared out or bulged near the contact
face. Slim-hole couplings are more susceptible to this
type of over-tightened damage than are full sized
couplings. Over-tightened full size couplings on Class D
and high strength sucker rods generally exhibit slight
bulges and have the concentric deformation ridge of
material on the coupling shoulder face from the
impression of the pin shoulder face. Over-tightening with
Figure 15
hydraulic rod tongs will twist off soft pins resulting in a
tensile failure appearance. The pin undercut will neck
down and fracture occurs rapidly. With Class D sucker rods, an indication of over-tightening is the concentric
deformation ridge of material on the pin shoulder face from the impression of the coupling shoulder face. Overtightening
on normalized and tempered high tensile strength sucker rods will begin to pull the threads out of the
coupling.
Figure 16
Figure 17
Figure 16 is an example of impact cracks on
couplings. The practice of “warming up,” or hammering,
on couplings in order to loosen them should not be
allowed. This example shows how impact damage to a
Class T coupling causes stress fatigue cracks around the
impact points and accelerated localized corrosion.
Hammering on Class SM couplings causes stress fatigue
cracks in the hard spray surface and results in a coupling
failure due to stress/corrosion fatigue.
Figure 17 is an example of polished rod failures.
The majority of all polished rod failures occur either in the
body, just below the polished rod clamp, or in the pin.
Polished rod body failures below the polished rod clamp
result from the addition of bending stresses. These
bending stresses may be imposed by pumping units out
of alignment, carrier bars that do not set level, worn
carrier bars, misaligned load cells, or incorrect polished
rod clamp installation. The polished rod failure on the left
is an example of polished rod clamp on the sprayed
portion of a Spraymetal polished rod. Spraymetal
polished rods have an unsprayed portion for polished rod
clamp placement. Never put a polished rod clamp on the
sprayed portion of a Spraymetal polished rod. The
polished rod failure on the right has small, longitudinal
scratches caused from mishandling.
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Polished rod pin failures generally occur due to the installation of sucker rod couplings. Polished rod
pins have a 9ϒ thread taper between the straight-threaded section and the shoulder. Sucker rod couplings have
a 30ϒ starting thread and a deep recess that doesn’t engage all the polished rod pin threads. Polished rod
couplings have a 9ϒ starting thread and a profile designed to properly fit the polished rod pin. The shallow
recess to the first thread easily distinguishes polished rod couplings from sucker rod couplings and allows every
polished rod pin thread to be engaged.
Corrosion Failures
Corrosion is one of the greatest problems encountered with produced fluids and accounts for about onehalf
of all sucker rod failures. Corrosion is the destructive result of an electrochemical reaction between the
steel used in making sucker rods and the operating environment to which it is subjected. Simply put, corrosion
is nature’s way of reverting a man-made material of a higher energy state (steel), back to its basic condition
(native ore) as it is found in nature. The elemental iron in steel combines with moisture or acids, to form other
compounds such as iron oxide, sulfide, carbonate, etc. Some form and concentration of water is present in all
wells considered corrosive and most contain considerable quantities of dissolved impurities and gases. For
instance, carbon dioxide (CO 2 ) and hydrogen sulfide (H 2 S) acid gases, common in most wells, are highly soluble
and readily dissolve in water, which tends to lower its pH. The corrosivity of the water is a function of the
amount of these two gases that are held in solution. All waters with low pH values are considered corrosive to
steel, with lower values representing greater acidity, or corrosiveness.
All downhole environments are corrosive to some degree. Some corrosive fluids may be considered
non-corrosive if the corrosion penetration rate, recorded as mils of thickness lost per year (mpy), is low enough
that it will not cause problems. However, most producing wells are plagued by corrosion problems and no
currently manufactured sucker rod can successfully withstand the effects of this corrosion alone. While
corrosion cannot be completely eliminated it is possible to control its reaction. All grades of sucker rods must be
adequately protected through the use of effective chemical inhibition programs (reference current editions of API
Specification 11BR and NACE Standard RPO195). Some sucker rod grades, due to different combinations of
alloying elements, microstructures and hardness levels, are capable of longer service life in inhibited corrosive
wells than other grades of either low or high tensile strength.
Why do new sucker rods seem to corrode faster than older rods in the same string? Two sucker rods
with the same chemical analysis will form a galvanic corrosion cell if the physical condition of one is different
from the other. Physical differences in a sucker rod may be caused from poor care and handling practices (i.e.
surface damage resulting in bruises, nicks, bends) and/or corrosion deposits. Since new sucker rods go into the
well without corrosion deposits, they often corrode preferentially in relation to rods that are coated with corrosion
deposits. Corrosion on steel starts very aggressively but often slows down as soon as an obstructive surface
film of corrosion deposit (scale) is formed upon the metal surface. For example, CO 2 generates iron carbonate
scale as a by-product of its corrosion. This scale coats the sucker rod and retards the corrosion penetration
rate, which tends to slow down corrosion. However, if this deposit is continuously cracked by a bending
movement or removed by abrasion, aggressive local corrosion continues on the area with the scale removed,
and results in deep corrosion pitting.
Can high tensile strength sucker rods be used in a corrosive environment? Generally soft rods tolerate
corrosion better than hard rods and, as a rule of thumb; you should always use the softest rod that will handle
the load. However, if load requirements dictate the use of high tensile strength rods than it is important to
protect the rods with an effective surface film of corrosion inhibitor. In most cases, if you can adequately protect
downhole equipment from corrosion, you should be able to adequately protect high tensile strength rods from
corrosion by increasing the application frequency of the corrosion-inhibitor program. In other words, if you
effectively batch treat once a week with 40 parts per million (ppm) of corrosion inhibitor for D class rods, you will
need to batch treat twice weekly with 40 ppm of corrosion inhibitor for high tensile strength rods. Treatment
volumes vary and are dependent upon many factors too numerous to list. Always consult with a corrosion
control specialist prior to the installation of every rod string, especially when corrosion fatigue is suspected as
the failure root cause.
9

Figure 18
Figure 18 is an example of corrosion fatigue from
CO 2 corrosion. The size of the pit, as far as when it
becomes detrimental to the rod, depends on two factorsmaterial
type and hardness. Class K sucker rods may
develop deeper and larger pits than a Class D sucker rod
before it becomes detrimental to the rods. Class D sucker
rods may develop deeper and large pits than a high
tensile strength rod before it becomes detrimental to the
rods. Softer materials with lower rod stress tolerate larger
pits than do harder materials with higher rod stress.
Therefore, small pits can be detrimental to higher tensile
strength sucker rods as opposed to a softer rod that does
not have as much rod stress.
Figure 5
Acid Corrosion
Service companies use acids for well
stimulation and cleanout work. All acid work should
have an inhibitor mixed with the acid prior to injection
into the well. Spent acids are still corrosive to steel and
the well should be “flushed“ long enough to recover all
acid. In rare instances, some produced waters contain
organic acids that have formed downhole, such as
acetic, hydrochloric and sulfuric acids. Corrosion from
acid is a general thinning of metal, leaving the surface
with the appearance of sharp, feathery or web-like
residual metal nodules. Metallic scale will not be formed
in the pits. The left sample in Figure 5 is an example of
acid corrosion.
Chloride Corrosion
Chlorides contribute to the likelihood of an increase in corrosion related sucker rod failures. The
corrosivity of water increases as the concentration of chlorides increase. Corrosion inhibitors have more
difficulty reaching and protecting the steel surface of sucker rods in wells with high concentrations of chlorides.
Corrosion, from waters with high concentrations of chlorides, has the tendency to be more aggressive to carbon
steel sucker rods than to alloy steel sucker rods. Chloride corrosion tends to evenly pit the entire surface area
of the sucker rod with shallow, flat-bottomed, irregular shaped pits. Pit shape characteristics include steep walls
and sharp pit edges.
CO 2 Corrosion
CO 2 combines with water to form carbonic acid, which lowers the pH of the water. Carbonic acid is very
aggressive to steel and results in large areas of rapid metal loss that can completely erode sucker rods and
couplings. The corrosion severity increases with increasing CO 2 partial pressure and temperature. CO 2
corrosion pits are round based, deep with steep walls and sharp edges. The pitting is usually interconnected in
long lines but will occasionally be singular and isolated. The pit bases will be filled with iron carbonate scale, a
loosely adhering gray deposit generated from CO 2 .
Figures 19 and 20 show typical examples of CO 2 corrosion. Figure 19 is an example of CO 2 corrosion
on couplings and Figure 20 is an example of CO 2 corrosion on rod bodies.
10

Figure 19
Figure 20
H 2 S Corrosion
Figure 21
H 2 S pitting is round based, deep with steep walls
and beveled edges. It is usually small, random, and
scattered over the entire surface of the rod. A second
corrodent generated by H 2 S is iron sulfide scale. The
surfaces of both the sucker rod and the pit will be covered
with the tightly adhering black scale. Iron sulfide scale is
highly insoluble and cathodic to steel which tends to
accelerate corrosion penetration rates. A third corroding
mechanism is hydrogen embrittlement, which causes the
fracture surface to have a brittle or granular appearance.
A crack initiation point may or may not be visible and a
fatigue portion may not be present on the fracture surface.
The shear tear of a hydrogen embrittlement failure is
immediate during fracture due to the absorption of
hydrogen and the loss of ductility in the steel. Although a
relatively weak acid, any measurable trace amount of H 2 S
is considered justification for chemical inhibition programs
when any measurable trace amount of water is also
present.
Figure 22
Figure 21 and 22 are examples of H 2 S corrosion. The
three rod body samples on the left are examples of
localized corrosion (pitting) and the two rod body samples
on the right are examples of general thinning corrosion
from under-scale deposit corrosion. The sample in Figure
22 is an example of a pin failure due to hydrogen
embrittlement.
Microbiologically Influenced Corrosion (MIC)
Some amount of microscopic life form is present
in essentially every producing well. Of primary concern to
sucker rods are the single celled organisms capable of
living in all sorts of conditions and multiplying with
incredible speed-commonly referred to as bacteria or "bugs”. Suspect fluids should be monitored continuously
for bacteria by sampling, identifying and counting the bacteria. The extinction dilution technique is commonly
used to culture bacteria for an estimation of the number of bacteria present in the well. Bactericide should be
11

used on all suspect fluids to control bacteria populations. Bacteria are classified according to their oxygen
requirements: aerobic (requires oxygen), anaerobic (no oxygen), and facultative (either). Some bacteria
generate H 2 S, produce organic acids or enzymes, oxidize soluble iron in produced waters, or any combination of
the preceding. MIC has the same basic pit shape characteristics of H 2 S, often with multiple stress cracks in the
pit base, tunneling around the pit edge and/or unusual
anomalies (i.e. shiny splotches) on the rod surface. Figure 23
Bacteria are very aggressive and all sucker rod grades
corrode rapidly in downhole environments containing
bacteria. Sulfate reducers (SRB’s), those that produce
H 2 S, probably cause more problems to downhole artificial
lift equipment than do any other bacteria type. Multiple
cracking in the pit bases results from the hydrogen sulfide
by-product of the bacterial lifestyle, which corrode and
embrittle the surface of the steel under the colony. Figure
23 show several examples of microbiologically influenced
corrosion (bacteria) on sucker rod bodies.
Oxygen Enhanced Corrosion
Oxygen enhanced corrosion will be most
prevalent on couplings, with a few instances found on rod
upsets. Oxygen enhanced corrosion is rarely seen on the
rod body. In fact, aggressive oxygen enhanced corrosion
can erode couplings without harming the sucker rods on
either side. The rate of oxygen enhanced corrosion is
directly proportional to the dissolved oxygen
concentration, chloride content of the produced water
and/or presence of other acid gases. Dissolved oxygen
can cause severe corrosion at extremely low
concentration and evaporate large amounts of metal.
Pitting is usually shallow, flat-bottomed, and broad-based
with the tendency of one pit to combine with another. Pit
shape characteristics may include sharp edges and steep
sides if accompanied by CO 2 or broad, smooth craters
with beveled edges if accompanied by H 2 S. Corrosion
rates increase with increased concentrations of dissolved
oxygen.
Figure 24
Figures 24 and 25 are examples of oxygen
enhanced corrosion. The coupling sample on the left is
an example of the effects of oxygen enhanced CO 2
corrosion (left), H 2 S corrosion (middle), and chloride
corrosion (right) while the rod samples in Figure 25 show
the effects of oxygen enhanced CO 2 corrosion near the
upset (left) and CO 2 corrosion on the rod body (right).
Figure 25
Scale Corrosion
Scales such as barium sulfate, calcium
carbonate, calcium sulfate, iron carbonate, iron oxide
(rust), iron sulfide, and strontium sulfate should be prevented from forming on sucker rods. Although scale on a
sucker rod slows down the corrosion penetration rate, it also reduces the effectiveness of chemical inhibitors.
Severe localized corrosion in the form of pitting results any time the scale is cracked by a bending movement or
removed by abrasion.
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Stray Current Corrosion
Rarely seen in most producing wells, stray current corrosion refers to the induced, or stray, electrical
currents that flow to or from the rod string. Stray current corrosion can be caused by grounding electrical
equipment to the well casing or from nearby cathodic protection systems. Arcs originating from sucker rods
leave a deep, irregular shaped pit with smooth sides, sharp edges and a small cone in the base of the pit. Arcs
originating from the tubing leave deep pits with smooth sides and sharp edges that are random in dimension
and irregular in shape. Stray current corrosion pits are usually singular and isolated in a row down one side of
the sucker rod near the upsets.
Manufacturing Defects
Failures due to manufacturing defects are rare
and seldom occur. Manufacturing defects are easily
recognized and it is important that you understand what
these defects look like if you are to file accurate claims for
warranty reimbursement. No manufacturer is excluded
from the possibility of defects in material or workmanship
and the following failure examples include defects from all
manufactures.
Figure 26
Figure 26 is an example of mill defects. Mill
defects occur along one side of the rod body and these
discontinuities normally have longitudinally tapered, sharp
“V” shaped bottoms with indications of the longitudinal seam in the base. The example on the far left is an
example of a sliver. The rod body third from the left is also an example of a sliver. When fishing the rod failure,
the sliver folded against the fracture surface. The rod body second from right is an example of a scab. A sliver
is a small loose or torn segment and a scab is a large loose or torn segment of material longitudinally rolled into
the surface of the bar. One end of the sliver or scab is normally metallurgically bonded into the rod body while
the remaining end is rolled into the surface and physically attached. Fatigue failures, which result from slivers or
scabs, will have a piece of loose material protruding over the fatigue portion of the fracture surface. The rod
body second from the left is an example of rolled-in-scale. Rolled-in-scale is a surface discontinuity caused
when scale (metal-oxide), formed during a prior heat, has not been removed prior to bar rolling. The rod body
sample on the far right is an example of a rolling lap. Rolling laps are longitudinal surface discontinuities that
have the appearance of a seam from rolling, with sharp corners folded over and rolled into the bar surface
without metallurgical bonding.
Figure 27 is an example of forging defects. The
fracture begins internally below a forging crack in the
upset area and is brittle or granular in appearance. A
crack initiation site may or may not be visible and a
fatigue portion may not be present on the fatigue fracture
surface. The examples on the left and in the middle occur
as a result of low forging temperatures. The example on
the left is a failure from cold-shut and the example in the
middle is a failure from a forging crack. The fracture on
the right is a failure caused by a subsurface longitudinal
seam located near the end of the raw bar. During the
forging process the orientation of this discontinuity was
changed transversely.
Figure 27
13

Figure 29 is an example of processing
defects. The lower example is a casehardened
sucker rod and the upper example is a coupling that
has been processed through a grinding operation to
reduce the diameter. In both examples, a difference
in the material hardness has resulted in preferential
corrosion attack.
Figure 30 is an example of a mill defect and
a machining defect. The lower example is a failure
due to a large, internal, nonmetallic inclusion in the
pin. The fracture began internally and is brittle or
granular in appearance. A crack initiation site may or
may not be visible and a fatigue portion may not be
present on the fatigue fracture surface. The upper
example is from rolling the pin threads twice. Rolling
twice has flattened the pin thread crest and will not
be capable of achieving the correct friction load
required for makeup.
Figure 29
Figure 30
Your initial investment in sucker rods is
substantial. Moreover, the cost related to replacing
damaged sucker rods generally out weights the
original cost of the new rod string. Protecting your
investment and getting the maximum service life out
of your rod string just makes good sense. It is
important to diagnose rod failures accurately and to implement corrective action measures to prevent future
failure occurrences. This photo essay is intended for use as a reference guide in sucker rod failure analysis. It
explains how rod failures occur and expounds methods for identifying the characteristics of the several failure
mechanisms. Where sucker rod failures are concerned, there are no absolutes and no two fractures look
exactly alike in appearance. But, by recognizing the visual clues and identifying characteristics of the different
failure mechanisms, corrective action measures can be taken to prevent sucker rod failures, thus allowing the
operator to produce marginally profitable wells more cost effectively.
14

For further information please contact:
Alberta Oil Tool
9530 – 60 th Avenue
Edmonton Alberta T6E 0C1
Phone: (780) 434-8566
Fax: (780) 436-4329
www.albertaoiltool.com
Issue Date: 10/30/01